Electrostatically-controlled resistance tube



R. FORMAN 2,935,624

ELECTROSTATICALLY-CONTROLLED RESISTANCE TUBE May 3, 1960 Filed Aug. 26, 1955' INVENTOR Unit fl States Pam-m ELECTROSTA'llCALLY-CONTROLLED RESISTANCE TUBE Ralph Forman, Hyattsville, Md., assignor to the United I States of America as represented by the Secretary of Commerce I Application August 26, 1955," Serial No. 530,915 1 Claim. c1. 307 -885) This invention relates to a variable resistance tube and more particularly to a tube employing a porous semiconductor, the resistance of which maybe-electrostatically controlled by anelectric potential applied to an ele- 7 ment surrounding or. adjacent to the semiconductor.

The present invention is a continuation-in-part of an application Serial Number 460,778 for a Magnetoresistive Device filed October 6, 1954 by RalphFormanand Robert G. Breckenridge which has now matured into US.

Patent No. 2,828,396.

' -It is wellknown that porous oxide coatings on various metals act as good electron emitters and have been used successfully as cathode material for many types of electron tubes.

The present invention is directed to the fact that these coatings exhibit a marked change in resistance when subjected to a variation in applied electrostatic field.

2,935,624 Patented May '3, 1960 2 23' are in contact with each other. The coatings are heated by means of filaments 24 and 25. Electfie current is supplied to coatings 23 and 23 through leads 26 and 27. metallic element 28 which is connected to a suitable potential source (not shown) byway of lead 29. An evacu ated glass envelope '3 0surrounds the entire structure;

Fig. 3 shows a circuit arrangement in which the resistance tube of the present invention is connected in the form of an amplifier. The oxide coating shown as 33 is supplied a plate current l by a source of voltage E A negative voltage or retarding potential in the manner of an amplifier grid voltage is applied to metalli'c'element 32 (in Fig. 3) adjacent or surrounding element 33, by variable source E The cha'racteristic curves for the device are shown in Figs. 4 and -5. The curves in Fig. 5 are very similar to those showing variation of plate current with grid voltage of an ordinary triode. The equivalent g of this tube, variation of I with a variation in E is in the order of a few hundred micromhos, and this tube can be used as a voltage amplifier or power amplifier. The advantages of this device are that it has a very high. impedance input, because of the negative voltages applied ,toelement 32 in Fig. "3, and'the output impedance can be made very low if the resistance of the material 33 is decreased by changing its dimensions. r Fig. 4 is a plot of the current l through coating 33' of I Fig. -3 as a function ofthe series voltage source E, for

three different values of electrostatic supply voltage B while Fig. 5 is a plot of I as a function of E for three different values of voltage source E 3 The'principle of operation of this device is based on apparent upon reference to the specification and drawings in which,

Fig. 1 shows one embodiment of an electrostatically-:

controlled resistance tube of the present invention;

Fig. 2 shows another embodiment in modified form of an electrostatically-controlled resistance tube of the present invention; 7

Fig. 3 shows the tube of either Fig. 1' or Fig. 2 arranged in a circuit to form an amplifier;

Figs. 4 and 5 show' transfer characteristics derived from the circuit of Fig. 3.

Referring to the drawings, in Fig- 1 is shown a high temperature ceramic base 1 composed of insulating material such as MgO on which is deposited an oxide cathode coating 2. Base 1 is hollow and a filament 8 shown in dotted lines is placed inside with external lead wires 4 and 5 coupling filament 8'out of base 1. Metal-ceramic contacts 3 provide electrical coupling to the cathode coating 2. Electric current is passed through coating 2 by means of leads 6 and 7. A metallic plate 9 supplied by lead 10 is positioned adjacent to coating 2 and serves to establish an electrostatic potential in the region of the coating. An evacuated glass envelope '11 completely surrounds the'tube structure.

Fig. 2 shows a second embodiment of the present invention. Numerals 23 and 23' refer to oxide coatings corresponding to the coating 2 of Fig. 1. Coatings 23 and 23 are deposited around the ends of nickel cylinders 21 and 22,'respectively. As shown, the two coatings are separated by a small air gap, but, if desired, cylinders 21 the theorydevelopedfor' porous semiconducto rs (oxide cathodes) which are good thermionic emitters. Athigh temperatures, for example, ranging ffom 700 K. to I100- K.; electrical conduction in the oxide cathode takes place because an electron gas exists inthe porous structure of the material." This mechanism assumes that electrons, thermionically-emit ted from the crystals, form an electron gas in the pores of the oxide cathode, and, at sufficiently high temperatures, current through the oxide cathode is predominately due to current flow through the electron gas. The electrical conductivity of the oxide cathodeis' then'found to be functionally dependent on the space charge distribution" of theeleetron gas in the pores; If an electrostatic potential i'sfapplied'to an element adjacent to or surrounding the cathode material, which does not necessarily draw any current from the emitting material, the space charge distribution in the pores changes and alters the electrical conductivity of the oxide cathode.

an increase in the resistance of the coating 33. In Fig.

4, for example, when the voltage E equals zero, the

plate current I is quite large. However, as the chargeproducing potential E, increases negatively as repreand 22may be butted together so that coatings 23 and Y 7 sented by E and E02, the plate current decreases. This shows that the current flowing through coating 33 for a constant applied voltage is proportional to the strength of the electrostatic field to which the coating is subjected. Obviously any means for changing the magnitude of this field will havea corresponding eifect on the resistance of coating 33.

A similar, though less pronounced change in resistance "is noted it a charged bar is placed near element 32 If Surrounding the coatings is an electrostatic A change in electrical conductivity V causes l in Fig. 3 to change for a constant applied volt- J negatively charged, the bar will cause the resistance of coating 33 to increase as the bar approaches element 32, and the resistance will subsequently decrease to its original value as the negatively charged bar is drawn away.

Source E provides a permanent stationary substitute for such a bar. The increase and decrease in negative charge on element 32 produces the same effect as a. negatively charged bar moving toward and away from the metallic element 32 to the inner surface of the element, resulting in a negative charge on its inner surface adjacent coating 33.. This nearby negative charge drives electrons around the constantly emitting coating 33 into the pores of the coating, thereby increasing the negative electron space charge in the pores of the coating. With an increased space charge in the pores fewer electrons accelerated by plate source E are able. to flow across the pores and hence a smaller current 1;, flows forv an equal voltage E A decreased plate current for equivalent plate voltage represents an increase inelfective resistance of coating 33. r

With variable voltage source E in place of a charged bar the theory of operation remains the same. The source E pushes a surplus of electrons from its negative terminal onto element 32, the electrons being distributed on both surfaces of the element. The negative charge on the inner surface of element 32 increases the space charge of electrons in the coating pores effectively increasing the resistance of thecoatingl. .Removal of the electrons from the. nearby inner surface of element.

32 restores the original space charge distribution in the pores of coating 33, thus restoring its resistance to the original value. I

The advantages of this device over conventional amplifiers are as follows:

(a) The device is an amplifier with a very high impedance input (retarding fields are applied :by-.control electrode, element; 32 in Fig. 3, therefore the currents to the control electrodeare very low).

wease made in construction and arrangement within the scope of the invention as defined in the appended claim.

What is claimed is:

An electrostatic variable resistance tubecomprising a hollow insulated base, a porous semiconductive coating on said base, filament means within said base for heating said coating to a temperature above 700 K., means for passing an electric current through said coating, metallic means adjacent said coating, and means for applying a variable electric potential to saidmetallic published 195.0.

(b The output impedance of the device can be varied over a wide range of values by controlling the resistance ofthe device. The resistance of the device; can be conmeans. 1

References Cited in the file of this patent 1 UNITED STATES PATENTS OTHER REFERENCES Electrons and Holes in Semiconductors, by Shockley,

. Hutner et al.: On the Interpretation of Conduction andThermionic Emission of (:Ba4r).0 Cathodes, pages 567-571, of Physical Review, vol. 78,,No. 5, June 1, 1950.

Hensley: On Electrical Properties of Porous Semiconductors, Journal of Applied Physics, October 1952, vol. 23,, No. 10, pages 1122-1129.

Young: Electrical, Conductivity and Thermelectric Power of (Ba-Sr)0 and BaO, Journal of AppliedPhysics, October 1952, vol. 23, No. 10, pages.- 1129-1138.

Henisch: .Book, Semi-Conducting Material, page 129,

May 2; 1.9.5.2.

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